The conventional pre-regulator portion of a switching power supply that operates directly off the AC mains must incorporate many components whose ratings are selected to permit operation at 220 VAC, even though the supply might never be used except at 110 VAC. In like fashion, the ultimate end might be a 5 V or 15 V DC supply, but in order to avoid a transformer the AC mains are rectified and applied to a filter, with the expectation that a subsequent switching power supply will preserve efficiency while keeping the overall size of the supply small. This approach results in significant excess energy being stored in the filter capacitor and never used, as well as a very high voltage rating for that filter capacitor. It also requires some very robust rectifiers to withstand the initial flow of current needed to charge the filter, especially at 220 VAC.
These disadvantages can be avoided by not allowing the filter capacitor to charge to a voltage that is greater than needed by the subsequent load. This can be arranged by placing a FET as a switch in series with the filter capacitor. In an ordinary supply the filter capacitor recharges nearly instantly to the instantaneous voltage produced by the rectifier, which means it gets charged to the peak voltage. In the instant supply the FET switch is opened as soon as the charge on the filter is adequate. A voltage triggered latch circuit is responsive to the rectified AC input, which is also applied through a decoupling diode to the filter capacitor. The latch supplies a control signal to the FET switch. A further sensing circuit can monitor the current charging the filter capacitor, and can trigger the latch to open the FET switch as needed to limit (by halting) the initial in-rush current during the initial application of AC power. The filter capacitor will eventually become sufficiently charged that the limiting action ceases. The decoupling diode decouples the rectified peaks provided by the rectifiers from the voltage on the filter capacitor. This allows the filter capacitor to charge whenever the rectified input has an instantaneous voltage greater than the capacitor's (and the FET switch is closed), but also allows the output waveform from the rectifier to fall to zero during the zero crossings of the AC input waveform. That fall to zero in turn resets the latch, so that the process of gating the charge to be accepted by the filter capacitor is repeated during the next half-cycle of the input AC waveform. Because the FET switch is either saturated or off, it dissipates relatively little power, allowing the gated charge power supply to be very efficient.